I came across the same error in the logs of an old Server 2003 box, and his thought process and conclusions were very helpful!  Initially I had decided to turn the feature off, but I didn’t like that answer.  In the end I changed my settings to 25 crashes in 5 minutes.  I know it’s a band aid and not the answer, be we are looking at moving this application to a new server in the very near future.

All of a sudden this Monday, our clients started getting “Service Unavailable” errors all of a sudden. I looked deeper in to it, and it turned out the Application Pool on IIS6 for the ASP .Net app had crashed. So I restarted it, and everything started working again.

But this started happening intermittently about twice daily. Big problem.

Source: Application pool crashing issue

Below is the exact error I’d been getting, the only changes were time and the process ID would obviously change.

Event Type: Warning
Event Source: W3SVC
Event Category: None
Event ID: 1009
Date: 3/21/2017
Time: 10:49:36 AM
User: N/A
Computer: ComputerName
Description:
A process serving application pool ‘DefaultAppPool’ terminated unexpectedly. The process id was ‘5996’. The process exit code was ‘0x80’.

For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.